The ability to detect ultrasonic sounds (frequencies over 20 kHz) is not limited to mammals among the vertebrates. Instead, American shad, a clupeid fish (related to herring) can detect. The shad ear is clearly involved in ultrasound detection, but the mechanism of ultrasound is detection is not known. The shad ear has a number of characteristics that are unique among vertebrates. Moreover, there is evidence that the support structure of the sensory epithelial of the utricle, the primary auditory endorgan in shad, may be functionally analogous to the high frequency regions of the mammalian organ of Corti. The proposed work is to do a detailed physiological and ultrastructural analysis of the ear and eighth nerve in order to determine how ultrasound is detected. It will also quantify the ultrasound detection capabilities of American shad and other-related species in order to get a better understanding of the kinds of signals that can be detected. Physiological studies will examine the response characteristics of each inner ear otolithic endorgan, and determine the ear region(s) involved with ultrasound detection. Neural responses from these regions will be quantified. Using extracellular and intracellular techniques. Ultrastructural and morphological approaches will be used to understand the structure of the ear, and the mechanical adaptations for ultrasound detection. Psychophysical studies at the University of Maryland will examine hearing capabilities at sonic and ultrasonic ranges, and characterize the ability of these animals to detect signals in noise. These studies will also help elucidate the precise mechanisms of ultrasound detection. Behavioral studies at Mote Marine Laboratory (Sarasota, Florida) will help characterize response to echolocation sounds, and help determine which species are response to ultrasound. These investigations will provide an opportunity to examine ultrasound detection in a highly accessible ear. This system has the potential of serving as a model system where ultrasonic region of the ear is amenable to investigation. This system has the potential for contributing to our understanding of how hair cell systems, and their innervating neurons, are able to respond to ultrasonic stimuli. Moreover, this system could be useful in analysis of how otolithic organs of the ear in mammals can potential contribute to hearing.